1. Field of the Invention
This invention relates in general to a method and apparatus for measuring the resistivity of material such as semiconductor material.
2. Prior Art
Arrangements for measuring the electrical conductivity of material such as semiconductor discs and particularly where the semiconductor disc is coupled capacitively to a resonant circuit which is driven by a high frequency generator so as to dampen the resonant circuit as a function of its conductivity and wherein a voltage measuring device is connected to the resonant circuit so as to determine the conductivity of the semiconductor disc are known. For example, such an arrangement is described in Zeitschrift for angewandte Physik, Volume 23 (1967), Issue 4, pages 268-270. As described in this article, a metal probe with a known probe frontal area is pressed with a thin film of water onto a silicon disc and is then pressed onto a metal plate. The probe is connected electrically with the capacitor of a resonant circuit and is connected through a second circuit via the film of water which acts as a high capacitance electrical connection over the area of the silicon disc which is engaged by the probe frontal area and over the metal plate to the coil of the resonant circuit. Depending upon the conductivity of the electrical effective area of the silicon disc, the resonant circuit will be attenuated. The dependent resonant voltage is measured by a voltage measuring device and serves as a measure of the conductivity of the disc area. In this known system for measuring the local conductivity it is necessary to shift the silicon disc and the electrical effective area which is between the probe frontal area and the metal plate. Shifting of the probe on the surface of the silicon disc will bring the same results if the disc rests firmly on the metal plate. Under certain conditions, however, the variations of the measured voltage correspond to the local conductivity. A proportionality results if on the one hand the resistance of the silicon disc is small in comparison to the resonant resistance of the resonating circuit but on the other hand is large in comparison to the attenuation resistance of the resonating circuit. Thus, the attenuation resistance has to be as small as possible if the utilizable measuring area is desired to be as large as possible.
This, however, implies that the quotient of the resonant blind resistance and the attenuation resistance of the resonant circuit should be as high as possible. However, the requirement for high local disintegration requires for exact measuring the conductivity a decrease of the frontal probe area. This, however, increases the influence of the coupling capacity of the water film on the measuring results. The electrical characteristics of the capacitor formed by the film of water cannot be neglected. Furthermore, since the silicon disc forms a galvanic element through the film of water with the metal probe, the conductivity of the film of water can be easily changed. Also, however, the characteristics of the capacitor formed through the water changes very quickly. Thus, apparatus according to the prior art does not give results which are reproducible and is subject to high local disintegration and of poor quality.